1. Field of the Invention
This invention relates to a process using chemical vapor deposition for the manufacture of multi-site curved surfaces, such as domes. In particular, this invention relates to an improvement in the process to reduce loss due to cracking caused by mechanical stresses resulting from the mismatch of the coefficients of thermal expansion (CTE) between the product and the mandrel plate.
2. Description of Related Art
Stress cracking of chemical vapor deposited CVD curved parts, such as domes, is due in part to the mismatch of the coefficients of thermal expansion (CTE) between the graphite forms and ZnS or ZnSe parts. Such cracking, if not prevented results in substantial loss (30% to 35%) due to damage to the manufactured parts.
In the manufacture of ZnS or ZnSe parts by chemical vapor deposition (CVD), a number of mandrel plates 10 are bolted together, as shown in FIG. 1. Typically four mandrel plates 10 are arranged in the form of a rectangular box. The mandrel plates 10 are normally made from graphite. A CVD furnace is charged with a zinc metal and the mandrel plates are placed in a CVD furnace. The furnace is then covered, sealed, and vacuum connections are attached in preparation for operation of the CVD process. The furnace is then heated to operating temperature (1200.degree.-1300.degree. F.) and a flow of reaction gas, H.sub.2 S or H.sub.2 Se, and an inert carrier gas, such as argon, is initiated. The CVD process continues until a sufficient depth of material is deposited, after which the furnace is cooled (approximately 24 hours) and the mandrel plates 10 containing the CVD deposited parts are unbolted and the parts are removed from the furnace. The parts are examined for cracks and the general appearance. Parts which are not within specification are discarded.
During the cooling process the graphite mandrel plates 10 change dimension only slightly, while the CVD deposited ZnS and ZnSe change dimensions to a greater degree. For example, the coefficient of thermal expansion for ZnS and ZnSe is approximately three times that of graphite. The difference in the change in dimension due to the difference in the coefficients of thermal expansion (CTE) results in mechanical stresses in the ZnS and ZnSe parts. These stresses are most apparent in the manufacture of curved surfaces such as domes or bowls. As shown in FIG. 1a; CVD domes 12 are deposited on a graphite mandrel plate 10. The domes 12 engage the female section 14 of the mandrel plate 10. Bridges 16 of chemical vapor deposited material cover the male sections 18 of mandrel plates 10 and connect adjacent domes 12. During the cool-down of the mandrel plate 10 and CVD domes 12, the graphite from which the mandrel plate 10 is made contracts slightly, while the CVD dome 12 contracts at approximately three times the rate of the mandrel plate 10. It is believed the mechanical stresses or "pinning" which is caused by the contraction of the material of the domes 12 and the bridge section 16 cause the deposited material to be pulled apart. The mechanical stresses appear in the finished part in the form of small cracks.
Normally, the loss due to stress cracking is from 30% to 35%, however, the loss may run as high as 90% under certain circumstances. The reduction or elimination of the effect of the difference in the coefficient of thermal expansion on the ZnS and ZnSe parts would result in a significant reduction in the percentage of loss. The difficulty in solving this problem is increased by the significant mass of the mandrel plates, CVD deposited parts, and the furnace, in addition to the high temperature at which a CVD furnace operates. Further, the inability to access the CVD deposited parts and mandrel plates immediately after completion of the CVD process adds to the difficulty in solving the problem.